This invention relates to a frequency-prepositioning device for an indirect frequency synthesizer. The invention is more particularly concerned with an indirect frequency synthesizer having switchable phase loops.
The device according to the invention permits the construction of an indirect frequency synthesizer which makes use of one or a number of phase control loops. The invention makes it possible to improve the rapidity of stabilization--or the time of acquisition--of the output frequency of the synthesizer.
A few constructional designs of a synthesizer will first be briefly recalled. A synthesizer is a device which serves to generate a sine-wave signal, the frequency of which is programmed by digital control. FIG. 1 is a simplified diagram of an indirect frequency synthesizer having a phase loop and based on the principle of control of a submultiple of the frequency of a voltage-controlled oscillator (VCO) in dependence on a stable reference frequency having good spectral purity. This synthesizer is constituted by a voltage-controlled oscillator (V.C.O.) 1 for delivering an output frequency f.sub.N which is dependent on the control voltage V.sub.N applied thereto. The output of oscillator 1 is connected to a scale-of-N frequency-divider circuit 2 (N being a whole number) which delivers an output signal at the frequency f.sub.N /N applied to a phase comparator 3 to which is also applied a so-called reference frequency f.sub.ref. Phase comparator 3 delivers a signal whose direct-current component is proportional to the phase difference existing between the two signals which are applied thereto. A low-pass filter 4 is connected to the output of said comparator for the purpose of removing the high components of the spectrum of the output signal of the phase comparator. This filtering operation prevents the control voltage V.sub.N applied to the oscillator 1 from producing a parasitic modulation of the output frequency f.sub.N. Finally, an amplifier 5 is placed between the filter 4 and oscillator 1 in order to provide the loop gain of the phase control loop of the synthesizer.
Since the switching time of the synthesizer is inversely proportional to the bandwidth of the loop, it is sought to reduce this switching time by increasing the bandwidth. The following different methods for the achievement of this objective are open to choice.
A first such bandwidth increasing method relies on increasing the reference frequency. Unfortunately, this method suffers from two technical limitations: first, since the reference frequency is made higher, the programmable divider must operate at higher frequencies. Second, in the case of a single-loop synthesizer, the reference frequency must be lower than the interval of frequency swing of the synthesizer.
A second method for increasing bandwidth relies on reducing the frequency f.sub.N to be divided, either by frequency transposition or by making use of multiple loops so that the output signal of one loop serves as the frequency of transposition in another loop.
Finally, a third method for increasing bandwidth relies on making use of frequency-prepositioning as shown in FIG. 2. When programming the division rank N, programming of a prepositioning voltage V.sub.NP is carried out at the same time, this voltage being added to the control voltage delivered by the control circuit. This solution minimizes the frequency deviation which has to be compensated by the control loop after a frequency switchover. However, this prepositioning operation cannot be accurate by reason of the non-linearities and dispersions of the voltage-frequency characteristics of the voltage-controlled oscillator as a function of time and also as a function of temperature.